Various energy conservation techniques are known and used to attempt to extend the duration during which a portable device can be used with a given portable power source such as a battery. For example, many such devices incorporate a so-called sleep mode of operation during which the device operates in a reduced power mode of operation to thereby reduce average power demand over time.
Some wireless communication devices at least attempt to incorporate such a reduced power mode of operation into their facilitating signaling protocols. For example, a well known 802.11 protocol supports a powering saving mode. During a normal mode of operation, the 802.11-compliant device is typically either actively transmitting or receiving. Just prior to entering a reduced power mode of operation the device transmits a short data packet to a corresponding access point to inform the access point that the device is entering a reduced power mode of operation. The access point then typically buffers any packets that are received by the access point for transmission to the device. During the reduced power mode of operation the device occasionally monitors the access point beacon frame transmissions at different wake-up times to thereby determine if the access point has any buffered packets for the device. Upon detecting a notice that buffered frames are available for transmission to the device, the device can transmit a short data packet to the access point to indicate that the device will now enter a normal mode of operation and will therefore be available to receive transmissions of the buffered packets.
Such a protocol will typically provide at least some savings, over time, of energy consumption by the 802.11-compliant device. Unfortunately, all expected savings are not always achieved. For example, to effect the above procedure, the device must transmit at least a short message to the access point. Also pursuant to the 802.11 protocol, however, the device must typically first ascertain that the channel is clear during a particular preceding window of interest by using a clear channel assessment process. When the channel is busy, the device must usually defer until the channel is clear for at least a given amount of time and must then further defer by going into a so-called back-off mode of operation. During the latter mode of operation, a random (or at least pseudo-random) selection occurs to determine a particular contention window slot to be used by the device to transmit its message. More particularly, the device must typically be in an active receive mode during this window of activity in order to perform the clear channel assessment process. This active receive mode can be of potentially great duration (depending upon how busy the channel is) and can contribute greatly to a consumption of power notwithstanding the power-saving intent of the overall methodology.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are typically not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.